Should I use a stereo or mono microphone with the Nikon D800/D800E?

The D800 has a mono mic built in. The signal is equally spread to both stereo channels in the recording.

On the side of the camera is a powered 3.5mm stereo mic input jack. If you plug in a mono mic, the signal will be audible only on one of the channels in the recording. You will have to use a converter plug or fix it in post processing.

If you use the headphones to monitor the sound of an external mono mic, you will hear most of the sound on one channel and some of the sound on the other, but in the recording it will be on one of the channels only.